1. Field of the Invention
This invention relates to multilayered ceramic substrates for mounting semiconductor or integrated circuit chips thereon for interconnection thereof. The invention also relates to a method for manufacturing the substrates, a composition for insulating substrate materials.
2. Description of the Prior Art
In the fabrication of multilayered ceramic substrates, it is usual to employ, as a conductor material, noble metals such as Au, Ag, Pd or mixtures thereof. Because of the expensiveness and a great variation of the cost of these noble metals, attempts have been made to use Cu electrode materials which are inexpensive and low in resistance and have good solderability.
A typical method of fabricating a multilayered ceramic substrate using Cu electrodes is one which comprises screen-printing a Cu paste on a sintered substrate such as of alumina to form a circuit pattern and, after drying, firing the printed substrate at a temperature which is lower than the melting point of Cu, e.g. 850.degree. to 950.degree. C. in a nitrogen atmosphere having a controlled oxygen partial pressure sufficient to cause the organic components in the conductor paste to be fully burnt out without oxidation of the copper. A dielectric paste is subsequently printed and fired under similar conditions. For obtaining a multilayered structure, the above procedure for the formation of the electrode pattern and the dielectric layer is repeated until a desired number of the layers are formed.
However, the use of the Cu paste involves several problems. One of the problems is a difficult in properly controlling the oxygen partial pressure in the firing atmosphere during the firing step. If oxygen is contained in excess in the atmosphere, Cu is oxidized. On the contrary, if the oxygen content is less, the organic components contained in the paste are not decomposed as desired and metallization cannot be appropriately attained. Another problem is that formation of the multilayered structure requires repetition of firing whenever the respective pastes are printed. Thus, the lead time is prolonged, thus leading to the rise in equipment costs.
U.S. Pat. No. 4,234,367 describes a method of making multilayered glass-ceramic structures having copper-based conductors. In the method, copper-based conductors are metallized by appropriate combinations of a nitrogen atmosphere and a hydrogen atmosphere containing water vapor. However, the precise atmospheric control of water vapor and hydrogen is minute and very difficult.
U.S. Pat. No. 4,679,320 sets forth a process for producing a multilayer ceramic circuit board wherein a thermally depolymerizable resin binder and fatty acid ethylene oxide adduct type deflocculants are used as organic components. In this patent, complete removal of the organic components by decomposition in a nitrogen atmosphere is stated as not easy. A similar process is also described in U.S. Pat. No. 4,504,339.
Moreover, U.S. Pat. No. 4,649,125 teaches a process wherein inorganic peroxides such as CaO.sub.2, SrO.sub.2, BaO.sub.2 and the like are added to glass compositions for multilayered substrates in order to facilitate burning out of organic binders upon firing. The oxygen emitted during decomposition of the peroxide is used to oxidize residual carbon as would be left when the organic binder is decomposed, thereby eliminating the organic binder in the form of CO.sub.2.
In all the processes, however, the atmospheres for firing have to be undesirably controlled depending upon the type and amount of glass or ceramic to be fired since the organic binders are burnt out during the course of firing.
On the other hand, U.S. Pat. No. 4,714,570 proposes a process of fabricating multilayered ceramic substrates which is different from those of the above U.S. patents. The process comprises a burning-out step, a reduction step and a densification or firing step. In this process, a metal oxide such as cupric oxide is used as a starting material for conductor and the burning-out step is effected in an oxidative atmosphere containing oxygen in amounts sufficient to burn out carbon atoms contained in the binder used while heating at a temperature sufficient for thermal decomposition of the organic binder inside the multilayered ceramic substrate. Following the burning-out step, the metal oxide is reduced to elemental metal in the reduction step, and sinterings of materials for the substrate and the elemental metal are conducted simultaneously to form an integral monolithic product in the densification step. As will be apparent from the above, the binder has been removed in the burning-out step, so that the atmospheric control for the sintering becomes very easy. In addition, since the organic binder has been completely eliminated, a dense sintered product with good insulating characteristics can be obtained irrespective of the amount of materials to be sintered.
However, the above process involves some problems. More particularly, the materials used as the multilayered ceramic substrate suffer contraction during the sintering process, which is significantly different from a rate of contraction of a metal such as copper used as the conductor material. This undesirably leads to warpage and deformation of the multilayered substrate. Especially, when copper is used as the conductor material, limitation is placed on the composition of the substrate material since copper is lower in temperature at which contraction or sintering commences. Another problem resides in wettability of solder to a Cu conductor pattern formed at the top layer. When the multilayered ceramic substrate is applied as a circuit board, this presents a serious problem. The wettability is influenced not only by the composition or type of conductor material, but also by the composition of the ceramic substrate material. This is because the ceramic substrate material contains glass components for the purpose of sintering or firing at low temperatures. The low softening point glass materials tend to exude, at the time of sintering, to the surface of the electrode formed as the top layer. Presumably, this is the reason why the solderability is impeded.
It will be noted that the substrate materials should generally have high folding endurance, good insulating properties and good dielectric characteristics and should be sintered within a short time at low temperatures of not higher than about 1000.degree. C.